Have lazy proxies keep their callbacks around and clean up their lambdas in the dtor

I believe after this CL we will be at a place where we just have to null out the
fTarget of a lazy proxy and it will reinstantiate itself.

Bug: skia:
Change-Id: I88fdc70e149eba4514a0823da99383583394005c
Reviewed-on: https://skia-review.googlesource.com/102021
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
7 files changed